Active Devices Flashcards
Silicon Dioxide
Good insulator
High permittivity
Large bandgap between the valence and conduction band
Stable molecule and can tolerate high temperatures
Resistant to many chemicals make it ideal for use in fabrication of semiconductor devices
Transistors
Three terminal semiconductor devices that can regulate current and voltage and can also act as a switch
What are the 3 terminals in a Field Effect Transistor
Drain, Source and a Gate
Gate is the control input
Basic operation of FETs
A voltage on a control input produces an electric field that affects the current between two other terminals
MOSFET
A metal oxide semiconductor field effect transistor - a FET with an insulated gate
Constructed with a body made of a P-type substrate with holes as its majority carriers
Two n type channels are etched into the top surface with electrons as the majority carriers - these become the source and the drain when metal electrodes are attached
A gate electrode is insulated from the region between the source and the drain by the gate oxide - Silicone dioxide insulating layer
Source electrode is connected internally to the body electrode
MOSFET Operating Principles
A depletion layer is formed at the P-N junction surrounding source and the drain
Depletion layer acts as an insulator between the source and the drain
When a potential is applied between the source and the drain electrode - the depletion layer restricts the flow of charge carriers and no current passes through the transistor
MOFSET is a voltage controlled current source
The silicon dioxide insulating layer metal oxide layer gives the MOSFET an extremely high input resistance - so high that the MOSFET draws neglible current from the input gate signal
MOFSET - Positive field at gate eletrode
V gs > V th - A positive voltage bias at the gate electrode sets up a positive electric field
I ds > 0 A - Current flows between the source and the drain via the inversion layer
I gs ~ 0 A
N Type minority carriers (electrons) attracted by the positive field migrate to the gate electrode and form a conductive N channel inversion layer between the source and the drain
MOFSET - No Field at Gate
When V gs = 0V - the field surrounding the gate electrode collapses and no current (I ds) flows between the source and the drain
I gs ~ 0 amp
I ds = 0 amp
V gs < V th - no field at gate electrode
When the gate field is turned off - the N type minority scatter throughout the p type substrate and collapse the inversion layer
MOFSET - Gate voltage is much greater than the threshold voltage
I ds»_space; 0 Amp - Larger current flows between the source and the drain
V gs > V th
A larger positive voltage bias at the gate electrode attracts more N-type minority carriers to the inversion layer
A larger inversion layer allows for a larger current to pass between the source and the gate
MOSFETs Operation Overview
MOSFET is a type of transistor that works by varying the width of a conducting N channel (inversion layer) along which charge carriers flow
3 Terminals - Gate, Source and Drain
In an n channel (NPN) Mosfet - charge carriers are electrons - enter at the source and exit via the drain
Conventional current is from drain to source (I ds)
The gate voltage (V gs) controls the thickness of the channel
A positive gate voltage larger than the threshold voltage (V th) attracts electrons towards the gate via the electric field generated
The greater the electric field - the thicker the inversion layer becomes - thus reducing the resistance between the drain and the source - channel is said to be enhanced and will allow a greater current (I ds) to flow
Creating a negative field at the gate electrode repels electrons increasing the resistance of the channel and reducing the current - channel is said to be depleted
Applying an input signal voltage to the gate (V gs) controls the drain source current (I ds) and hence the output in the external circuit
The insulating metal oxide layer gives the MOSFET an extremely high input resistance - so high that the MOSFET draws neglible current from the input signal
It draws very little power from the input signal when operating as an amplifier
Metal oxide layer is extremely thin - MOSFET is susceptible to destruction by electrostatic charges building up on the oxide layer between the gate and the source (behaves like a capacitor) - gate should never be left unconnected - a path to ground is needed to allow the charge to flow off
P Channel MOSFET
Type of MOSFET in which the channel of the MOSFET is composed of a majority of holes as charge carriers
When the MOSFET is activated and is on, the majority of the current flowing are holes moving through the channels
MOSFET Output Characteristics when V gs < V th
When V gs is less than V th - MOSFET is off because there is conducting channel
Small leakage current flows, of the order of a few nanoamps
Above V th a channel starts to form and the MOSFET turns on
I ds versus V ds characteristic curves have almost vertical and almost horizontal parts - linear almost vertical part of the curves correspond to the ohmic region - were the MOSFET channel acts like a resistor
Linear region - above V th the drain current I ds increases slowly at first with an increase in V gs and then much more rapidly
The horizontal part corresponds to the constant current region - where there is almost no increase in drain current for increasing V ds - this is the saturation region - drain-source current is then controlled by the value of V gs
Summary of operating regions
Cut off region - with V gs < V th - gate-source voltage is lower than threshold voltage - so the MOSFET is switched off and I ds = 0 - MOSFET acts as if it was an open circuit
Ohm’s Law Region - with V gs > V th - the MOSFET acts like a variable resistor whose value is determined by the gate voltage V gs - up to the point where it becomes saturated
Saturation Region - with V gs > V th and a high enough value of V ds - the MOSFET is in its constant current region and is switched fully on or saturated - the current I ds is at its maximum and depends on the value of V gs
Transconductance (g m)
measured in units of mAV^-1
Change in drain current caused by the change in the voltage between the gate and the source
Calculating the gradient of the ohmic part of the graph
g m = Change in I d / change in V gs
Breakdown Region
Above saturation region is the breakdown region
At a certain value of V ds called the breakdown voltage - drain-source path of the MOSFET breaks down internally and a large current will flow - destroying the transistor
Application of MOSFETS
Switches
Intrinsic Semiconductor
Has no dopants present
Number of charge carriers is determined by the properties of the material itself instead of the amount of impurities
Insulator having a complete electron shell
Can create electron hole pairs resulting conduction
Elemental intrinsic semiconductors
Composed of single species of atoms
Compound semiconductors
Semiconductors are composed of two or more elements
P type extrinsic semiconductor
Boron atom will be involved in covalent bonds with three of the four neighboring Si atoms. The fourth bond will be missing and electron, giving the atom a ‘hole’ that can accept an electron
N type extrinsic semiconductor
A phosphorus atom with five electrons in the outer shell introduces an extra electron into the lattice as compared with the silicon atom
PN Junction
Due to diffusion, electrons move from n to p side and holes from p to n-side
Causes depletion zone at junction where immobile ion cores remain
Results in a built in electric field - which opposes further diffusion
Fermi levels are aligned across on junction under equilibrium
PN Junction Forward and Reverse Bias
Forward Biased - if the pd across the diode is high enough - the valence electrons in the depletion zone have enough energy to move freely - depletion zone disappears and current moves across the diode - diode is conducting in this state
Reverse Bias - if the voltage is high enough, at a breakdown voltage the depletion region breaks down and a very high reverse avalanche current flows - the breakdown is permanent and the diode is damaged
Active Components vs Passive Components
Active components require electrical energy to operate and can introduce energy to a circuit
Passive components can not introduce net energy to a circuit
LEDs
Semiconductor pn junction diode that emits light when in forward bias
Emit narrow bands of light
Electron from donor material recombines with hole in acceptor material
Produces photon with energy hf equal to that of the band gap
Smaller band gaps give infrared/red light - larger band gaps give blue/UV light
Plancks Constant and LEDs
Electrons in the semiconductor is raised above the equilibrium value by the electrical input energy
Most of these energy carriers give up their energy as spontaneous emission of photons sith energy equal to the bandgap of the semiconductor
Egap = hf
Diodes as light detectors
Photodiodes - turns light into a voltage or current signal - work on the principle of photo generation - only on or off
LDRs - resistance changes when light falls upon it - bidirectional
Photodiode
Diode that converts photon into voltage or current
When an electron absorbs a photon of sufficient energy - the electron moves to the conduction band - creating an electron hole pair
The photo generated charge migrates to the depletion region where it recombines with ions - this changes the voltage across the depletion region - magnitude of the change gets converted into charge and so into number of electrons
A second way to count photo generated charge is by monitoring the current through a reverse biased diode - leakage current will increase proportionately to the amount of photo-generated charge
Photoconductive mode
When a photodiode is connected in reverse bias
Photosensitivity
Spectral response of a photodiode
photosensitivity = current generated / power incident = Amps / Watt
Use of Photodiodes
Light meters
Smoke detectors
Position sensors
Photocopiers
Light detectors
Optical fibre communication systems
Smoke Detectors
Pulsing infrared LED is located in a chamber with a photodiode
Chamber is designed to exclude light from any external source
A photodiode is positioned in the chamber so that normally it does not intercept the beam from the LED
If smoke from a fire enters the chamber - the light from the LED will be scattered and some will be directed to the photodiode
This generates a photocurrent that can be amplified to sound an alarm
Photodiodes and Scintillators
Scintillators produce a flash of light when a particle such as an ion, electron, alpha particle or high energy photon passes through it
If coupled to a photodiode - number of light pulses can be detected and amplified - energy of the particle passing through the scintillator can be measured
Materials used to make the scintillator may be liquid or solid
Scintillator efficiencies tend to be low (3-15%)
A single particle event is capable of depositing enough energy in the scintillator to produce a few thousand photons
This is known as the light yield - measured as the number of photons per MeV
Photons produced in the scintillator typically have an energy of 3-4eV. This is enough energy to create an electron-hole pair in the depletion region of the photodiode
For maximum detection efficiency, the photosensitivity of the photodiode needs to be matched as closely as possible to the wavelength produced by the scintillator
Electrons are collected at the anode and the holes by the cathode of the diode
Charge collected is amplified and the pulse produced can be counted
Applications of Zener Diodes
Constant voltage source
Reference voltage
Zener Diodes as Voltage Regulators
Allows current to flow in the forward direction - also allows it to flow in the reverse direction when the reverse bias voltage across the diode is above a certain value
When connected in reverse bias and the applied voltage is increased - large reverse current at breakdown does not damage the diode because of the Zener diode’s special construction
Manufactured to have their reverse breakdown occur at a specific, well-defined voltage - designed to operate continuously in breakdown mode
Zener in a voltage regulator circuit
In reverse bias it is connected to the positive source terminal
The Hall Effect
When an electric current flows through a conductor within a magnetic field - magnetic field exerts a transverse force on the moving charge carriers
This force pushes the charges to one side of the conductor
A build up of charge at the sides of the conductors will balance this magnetic influence - produces a measurable voltage between the two sides of the conductor - Hall Voltage
Presence of this measurable transverse voltage is called the Hall effect
Hall Effect in Semiconductors
Thin piece of p type semiconductor material passing a continuous current through itself
Magnetic flux lines exert a force on the semiconductor material which deflects the charge carriers, electrons and holes, to either side of the semiconductor slab
A potential difference is produced between the two sides of the semiconductor material by the build up of these charge carriers - The Hall Voltage
Applications of Hall Effect Sensors
Tachometers - measure rotation spped of wheels or rotating machinery
Joystick - stick has a magnet in its base - gives rise to a varying voltage that is dependent on the orientation of the stick
